A known electrostatic recording film consists of an insulating film, a conductive layer and a dielectric layer which are laminated in this order. An electrostatic recording system comprises applying a recording voltage to a multipin electrode head (hereinafter simply called a "pin electrode"), inducing arc discharge within a fine void (hereinafter simply called a "gap") between the pin electrode and the dielectric layer of an electrostatic recording film to thereby form an electrostatic latent image and then developing the electrostatic latent image with the use of a toner so as to give a visible image.
In order to obtain a clear image, it is required to control the gap within an appropriate range based on Paschen's curve. The most common method for achieving the above-mentioned object comprises contacting a dielectric layer, to which insulating grains have been added so as to give an appropriately uneven surface, with a pin electrode to thereby appropriately control the gap. In the case of the above-mentioned electrostatic recording film, however, a clear image can never be obtained unless insulating grains are added to the dielectric layer. On the other hand, it is known that incomplete grounding of the dielectric layer would cause "fog".
In the case of a conventional electrostatic recording paper, grounding from the paper side of a conductive paper is possible. In the case of an electrostatic recording film comprising an insulating film, however, it is impossible to ground from the both sides of the insulating film. Therefore it has been attempted to expose some part (usually an end) of the conductive layer or to apply a conductive coating such as a carbon coating onto the exposed part to thereby form a grounding electrode. In these cases, however, an additional process for exposing the conductive layer corresponding to the width of each product or for applying the conductive coating is required, which would lower the production efficiency.
Therefore JP-B-57-12144 proposes an electrostatic recording film wherein a conductive grains are dispersed in a dielectric layer in such a manner that these conductive grains are come in contact with each other when a pressure of a definite level or above is applied, thus giving conductivity. (The term "JP-B" as used herein means an "examined Japanese patent publication".) According to JP-B-57-12144, the electrostatic recording film is charged with a pin electrode (pressure: 50 to 100 g/cm.sup.2) and then pressed with a conductive roll (pressure: 500 to 5000 g/cm.sup.2) prior to the development. Thus the conductive grains dispersed in the dielectric layer are come in contact with each other so as to keep the dielectric layer as to serve as a grounding electrode, thus solving the problem of fog.
Although such an electrostatic recording film shows no fog, a large amount of conductive grains should be added in order to achieve the contact of these grains with each other by applying pressure. Furthermore, this electrostatic recording film suffers from additional problems such as linear dislocation of pixels in the direction parallel to the recording electrode (hereinafter simply called "line dislocation") and an increase in enlarged pixels caused by abnormal discharge (hereinafter simply called "spotting").
Recently, JP-A-61-213851 proposes an electrostatic recording film wherein the above-mentioned disadvantages of the electrostatic recording film of JP-B-57-12144 are overcome. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) In the electrostatic recording film of JP-A-61-213851, conductive fine grains are added in such a manner that they are never contacted with each other to thereby prevent line dislocation and spotting.
Further, JP-A-57-101841 discloses the electrostatic recording film wherein the conductive fine grains are incorporated into the dielectric layer in order to improve the stability of corona discharging or recording property with high frequency.
However neither the conductive fine grains described in JP-B-57-12144 nor those described in JP-A-61-21385 can show a satisfactory reproducibility of fine lines which are the most important in drawings. Thus it has been required to overcome the problem of line cutout. Recent demand for a lower cost requires high-speed coating of an insulating layer. When the above-mentioned insulating grains comprise an inorganic substance such as calcium carbonate, the insulating grains frequently have a specific gravity of 1.0 or more. In such a case, therefore, the specific gravity of the coating solution should be elevated corresponding to the inorganic substance to thereby prevent the sedimentation of the insulating grains. The increase in the specific gravity of the coating solution results in an increase in the viscosity thereof, which makes high-speed coating impossible.
From these points of view, a polymer having a relatively low specific gravity is sometimes used as insulating grains.
However such a system would frequently suffer from the above-mentioned line cutout. Thus it has been urgently required to overcome this problem.